In an electronic device in which a semiconductor element, such as a transistor, is electrically connected to a substrate, it has been known to arrange a heat sink, such as a heat radiating member, opposite to the substrate with respect to the semiconductor element, and to separate an electrical connection path from the semiconductor element to the substrate and a heat radiation path from the semiconductor element to the heat sink from each other. For example, JP2002-50722A discloses such an electronic device, and teaches to fill a heat radiation path between the semiconductor element and the heat sink with a gel heat conductive material.
As a specific example of spacing the substrate from the heat sink by a predetermined space, projections are formed on the heat sink at locations corresponding to corners of a rectangular shape of the substrate as support portions, and the substrate are fixed to the support potions by screws or the like.
In such a structure, however, an intermediate portion of the substrate between the support portions is easily bent or deformed relative to the portions of the substrate corresponding to the support portions due to a change in temperature. Therefore, it is necessary to increase the thickness of electrically insulating and heat radiating material (heat conductive material) so as to restrict the semiconductor element from being in contact with the heat sink, causing an insulation failure, even when the substrate is deformed toward the heat sink. However, if the thickness of the electrically insulating and heat radiating material is excessively increased, heat radiation performance is likely to be degraded.
In an electronic device in which a plurality of semiconductor elements is mounted on a substrate, it is not always true that the semiconductor elements generate the same amount of heat when being electrically conducted. That is, there is a case where an element that generates a relatively large amount of heat and an element that generates a relatively small amount of heat are mounted on the same substrate. A technical idea of employing different heat radiation structures for such two kinds of elements has not been known.